Generally, a quartz optical fiber is produced by a modified chemical deposition method (hereinafter referred to as "MCVD" method) or a vapor phase axial deposition method (hereinafter referred to as "VAD" method). The MCVD method comprises forming a cladding layer and a core glass successively on an inner surface of tube made of natural quartz. Therefore, the outermost layer of the optical fiber consists of natural quartz which contains chlorine in an amount less than the limit of detection. Generally, in the VAD method, synthesized core glass is covered with a natural quartz tube so that the outermost layer of the optical fiber contains chlorine in an amount less than the limit of detection.
The quartz optical fibers produced by the above described methods have, however, such poor strength that they have an average broken length of 5 km in a tensile test at elongation of 0.7 %. The poor strength of the optical fiber is due to surface defects.
To eliminate or reduce the probability of break of the optical fiber due to the presence of foreign materials or impurities, it has been proposed to replace the natural quartz tube with a synthetic quartz tube in order to decrease the content of impurities in the optical fiber, or to use a covering made of a glass having a low coefficient of thermal expansion such as TiO-SiO.sub.2 type glass. However, such techniques have not been industrially employed.